The present invention relates generally to a xerographic unit and, more particularly, to a penta-level xerographic unit which produces five voltage levels on a photoreceptor with both Charge Area Development (CAD) and Discharge Area Development (DAD) for depositing toner of five colors.
In the practice of conventional bi-level xerography, it is the general procedure to form electrostatic latent images on a charge retentive surface such as a photoconductive member by first uniformly charging the charge retentive surface. The electrostatic charge is selectively dissipated in accordance with a pattern of activating radiation, such as a light beam, corresponding to original images. The selective dissipation of the charge leaves a bi-level latent charge pattern on the imaging surface where the high charge regions correspond to the areas not exposed by radiation. One level of this charge pattern is made visible by developing it with toner. The toner is generally a colored powder that adheres to the charge pattern by electrostatic attraction. The developed image is then fixed to the imaging surface, or is transferred to a receiving substrate such as plain paper, to which it is fixed by suitable fusing techniques.
In tri-level, highlight color imaging, unlike conventional xerography, upon exposure, three charge levels are produced on the charge-retentive surface. The highly charged (i.e. unexposed) areas are developed with toner, and the area more fully discharged is also developed, but with a toner of a different color. The area with an intermediate exposure is not developed. Thus, the charge retentive surface contains three exposure levels; zero exposure, intermediate exposure, and full exposure, which correspond to three charge levels. These three levels can be developed to print, for example, black, white, and a single color.
In the tri-level xerographic unit, as illustrated in prior art FIGS. 1A and 1B, a photoconductive belt consisting of a photoconductive surface on an electrically conductive, light-transmissive substrate is charged to a selectively high uniform positive or negative potential, V.sub.0.
The uniformly charged surface of the photoconductive belt is exposed by a tri-level raster output scanner (ROS), which causes the charge retentive surface to be discharged in accordance with the output from the scanning device. This scan results in three separate discharge regions on the photoreceptor, each region exposed at one of three possible levels: (1) zero exposure which results in a voltage equal to V.sub.ddp and will be developed using charged-area-development (CAD); (2) full exposure, which results in a low voltage level V.sub.C and is developed using discharged-area-development (DAD); and (3) intermediate exposure, which yields an intermediate voltage level V.sub.W and does not develop and yields a white region on the print. These voltage levels are shown schematically in FIGS. 1A and 1B.
The photoreceptor, which is initially charged to a voltage V.sub.0, undergoes dark decay to a level V.sub.ddp (V.sub.CAD) equal to about minus 900 volts in this illustrative example. When exposed at an exposure station, the photoreceptor is discharged to V.sub.c, (V.sub.DAD) equal to about minus 100 volts in the highlight (i.e. color other than black) color portions of the image. The photoreceptor is also discharged to V.sub.w (V.sub.white) equal to minus 500 volts imagewise in the background (i.e. white), image areas and in the inter-document area. Thus the image exposure is at three levels; zero exposure (i.e. black), intermediate exposure (white) and full exposure (i.e. color). After passing through the exposure station, the photoreceptor contains highly charged areas and fully discharged areas which correspond to CAD and DAD color latent images, and also contains an intermediate level charged area that is not developed.
A development system advances developer materials into contact with the CAD and DAD electrostatic latent images. The development system in a tri-level xerographic unit comprises a first and second developer. The first developer brings developer material, by way of example, positively charged black toner, into contact with the photoreceptor for developing the charged-area regions (V.sub.CAD). A suitable DC electrical bias, V.sub.bb, of approximately minus 600 volts is applied to the first developer.
The second developer brings developer material, by way of example, negatively charged red toner, into contact with the photoreceptor for developing the discharged-area regions (V.sub.DAD). A suitable DC bias, V.sub.cb, of approximately minus 400 volts is applied to the second developer.
An illustrative example of a tri-level xerographic unit is found in U.S. Pat. No. 4,990,955, assigned to the same assignee as the present invention and herein incorporated by reference.
There are several scanning techniques known in the prior art to obtain the tri-level exposure imaging. A conventional flying spot scanner, such as used in the Canon 9030 uses a ROS unit to "write" an exposed image on a photoreceptive surface a pixel at a time. To obtain higher spatial resolution, a pulse imaging scanner can be utilized. This pulse imaging scanner is also referred to as a Scophony scanner in an article in Optical Engineering, Vol. 24, No. 1, January/February 1985, Scophony Spatial Light Modulator, by Richard Johnson et al., whose contents are hereby incorporated by reference. A preferred technique, capable of higher spatial resolution is to use similar optical elements as the flying spot scanner (rotating polygon, laser light source, pre-polygon and post-polygon optics), but with an A/O modulator which illuminates many pixels at a given time, resulting in a scanner with a coherent imaging response. With this type of scan system, the exposure level, or levels at the image surface, can be controlled by controlling the drive level of the A/O modulator dependent on the video data. In a tri-level system, two drive levels are used, one for the white exposure and a second higher drive level for the DAD exposure.
Alternately, instead of obtaining an intermediate exposure level by controlling the acoustic amplitude, an intermediate exposure can be provided by using pulse width modulation in a pulse imaging system in conjunction with spatial filtering.
In quad-level or four-level color imaging, upon exposure, four charge levels are produced on the charge-retentive surface. Thus, the charge retentive surface contains four exposure levels; zero exposure, a low intermediate exposure, a high intermediate exposure and full exposure, which correspond to the four charge levels. These three levels can be developed to print, for example, black, white, and two colors.
In the quad-level xerographic unit, as illustrated in prior art FIG. 2, a photoconductive belt consisting of a photoconductive surface on an electrically conductive, light-transmissive substrate is charged to a selectively high uniform positive or negative potential, V.sub.0.
The uniformly charged surface of the photoconductive belt is exposed by a quad-level raster output scanner (ROS), which causes the charge retentive surface to be discharged in accordance with the output from the scanning device.
The photoconductive belt, which is initially charged to a voltage V.sub.0 (approximately minus 1000 volts), is discharged to V.sub.w (approximately minus 700 volts) imagewise in the background (white) image areas and to V.sub.d (approximately minus 350 volts) and V.sub.a (approximately minus 100 volts) in the highlight (i.e. colors other than black)image areas.
A development station advances developer materials into contact with the electrostatic latent images on the photoconductive belt. The development system in a quad-level xerographic unit comprises a first, second and third developer.
The black toner from the first developer is attracted to the V.sub.0 voltage areas on the photoreceptor and repelled from the other two charged areas, V.sub.d and V.sub.a. The positively charged black toner from the first developer is attracted to the V.sub.0 voltage areas on the photoreceptor belt which are at a charge level of minus 1000 volts since the bias on the first developer is minus 800 volts. The positively charged black toner is attracted to the photoreceptor areas which are more negative than the developer housing. Conversely the positively charged black toner from the first developer housing is not attracted to the photoreceptor areas, V.sub.d (approximately minus 350 volts) and V.sub.a (approximately minus 100 volts), that are more positive than the first developer housing bias of minus 800 volts.
The magenta toner from the second developer is attracted to the V.sub.a voltage areas on the photoreceptor and repelled from the other two charged areas, V.sub.d and V.sub.0. The voltage level V.sub.a of minus 100 volts is less negative than the minus 300 volts of the second developer housing and the negative charge of the magenta toner. The magenta toner is not attracted to the photoreceptors areas of voltage levels V.sub.d of minus 350 volts because these areas are more negative than the minus 300 volts bias of the second developer housing and thus repel the magenta toner.
The cyan toner from the third developer is attracted to both the V.sub.a and the V.sub.d voltage areas on the photoreceptor. The voltage levels of V.sub.d of minus 350 volts and V.sub.a of minus 100 volts are both more positive than the minus 600 volts bias of the third developer and the negatively charged cyan toner.
Thus, the V.sub.0 voltage areas on the photoreceptor attracts the black toner from the first developer to produce a black color image. The V.sub.d voltage areas on the photoreceptor attracts the cyan toner from the third developer housing 130 to produce a cyan color image. The V.sub.a voltage areas photoreceptor attracts the magenta toner from the second developer and the cyan toner from the third developer to produce a blue color image. The areas of the photoreceptor charged to V.sub.w of minus 700 volts are not developed by any of the toners because the biasing of the toner housings and the polarities of the toners.
Thus, the quad-level xerographic unit, where the voltages of the color highlight areas on the photoreceptor and the color developer biases are between the white voltage level and ground, will produce black, white, cyan (the color of the toner whose housing bias is closest to white) and blue (a mixture of cyan and magenta).
An illustrative example of a quad-level xerographic unit is found in U.S. Pat. Nos. 4,731,634 and 5,155,541, commonly assigned with this application and herein incorporated by reference.
A quad-level xerographic unit, unlike the bi-level and tri-level, may not produce color images that match the toner colors. Two of the toner colors are produced while the third color produced is a combination of one of those first two toner colors and a third toner color.
There are alternate quad-level xerographic units for carrying out the desired formation of three different color pixels on the photoreceptor means of the present invention. Some of these alternatives, such as U.S. Pat. No. 5,049,949, assigned to the same assignee as the present invention and herein incorporated by reference, do not use the combining of two color toners to form a third color pixel on the photoreceptor means, but rather directly deposit three different color toners upon the photoreceptor means without combination.
It is an object of this invention to provide a penta-level xerographic unit for produces five voltage levels on a photoreceptor with both Charge Area Development (CAD) and Discharge Area Development (DAD) for depositing toner of five colors.